1. Field of the Invention
The present invention relates to an identification method of magnetic markers which are attached to articles to identify their kinds or to count the number thereof.
2. Description of the Prior Art
Attaching magnetic materials to articles and detecting the materials by using an alternating magnetic field to identify the article and to prevent them from being stolen is a known technique. In other words, the magnetic materials can be used as magnetic markers. For example, an article incorporating markers is prepared, the markers being made of such material as amorphous magnetic thin wires or thin bands, which have highly rectangular magnetic hysteresis curves. When the article passes through an alternating magnetic field, the field induces the magnetization inversion in the magnetic wires or bands. The magnetization inversion in turn produces the change in the magnetic flux, which can be detected by a detection coil externally provided. Since the amorphous magnetic materials exhibit excellent softiron magnetic characteristics which prior materials do not possess, magnetic markers of high sensitivity and small size can be achieved by the amorphous materials. Moreover, combining and depositing a plurality of amorphous magnetic thin wires or thin bands of different coercive forces makes it possible to identify magnetic markers and therefore the kind of articles. This is achieved because the amplitudes of the magnetic field at which the magnetization inversions occur vary depending on the magnetic materials, and so the magnetization inversions are detected as a temporal pulse train by using, for example, a triangular waveform or sinusoidal waveform magnetic field. This method identifies the kind of articles either by detecting the presence or absence of a pulse, or by detecting the level of pulses: the former detects the presence or absence of pulses at the expected phases of the alternating magnetic field corresponding to the coercive force of each magnetic material, that is, at the expected timing corresponding to these phases; the latter detects the voltage levels of respective pulses at given phases of the alternating magnetic field.
Although these two methods are effective, it sometimes occurs that the identification of the magnetic markers or the articles cannot be carried out. This is because the phases at which the pulses are generated by the magnetization inversion vary according to the difference of peak values of the alternating magnetic fields that magnetize the magnetic wires, and the peak values vary owing to the distribution of the alternating magnetic field in the region through which the markers pass, and to the direction in which the magnetic wires pass through the magnetic field.